1. Field of the Invention
This invention relates to processes, methods and systems for the treatment of wastewater, for example, domestic wastewater, which is contaminated with phosphorus. In particular, this invention relates to the chemical removal of undesirable chemicals, e.g., phosphorous, from wastewater.
2. Description of the Prior Art
The presence of phosphorus in sewage effluents has been recognized as promoting the growth of algae and aquatic plants in receiving waters by providing a source of nutrition. Phosphorus can be found in sewage in a number of forms, for instance, as soluble, insoluble or complex phosphorous including within organic matter.
Generally, phosphorous is removed from wastewater by chemical precipitation using salts of multivalent metal ions. The most common multivalent metal ions used are calcium, aluminum and iron. Calcium is usually used in the form of lime (calcium hydroxide), aluminum in the form of alum (aluminum sulphate), and iron in the form of ferric chloride. In addition to using a source of multivalent metal ions, polymers are also sometimes added to aid in the formation of flocs. The precipitation/flocculation process is generally followed by the removal of the precipitated/flocculated phosphorous using mechanical means such as by sedimentation or filtration. More recently, polymeric aluminum products such as PASS (polyaluminum silicate-sulphate), PASiC (aluminum-silicate polymer composite), and PAC (hydroxylated polyaluminum chloride) have also been patented (PASS) and tested (PASiC and PAC) in full-scale applications (PASS) and laboratory experiments (PASiC and PAC). The PASS formula, as an example, is defined in U.S. Pat. No. 5,149,400 patented Sep. 22, 1992 by Haase et al as:AlA(OH)B(SO4)C(SiOX)D(H2O)E
where A is 1.0, B ranges from 0.75 to 2.0, C ranges from 0.3 to 1.12, D ranges from 0.005 to 0.1, X is greater than 2.0 but less than or equal to 4.0 such as 3=B+2C+2D(X−2), and E is larger than 4 when the product is in aqueous form.
The use of these coagulants/flocculents is generally followed by removal of the resulting precipitate, e.g., by sedimentation or by filtration.
Many patents have been issued which are directed to the removal of phosphorus from wastewater. The more relevant patents are believed to be the following:
U.S. Pat. No. 4,981,675, patented Jan. 1, 1991 by Handy Chemicals Inc., which provided water treatment methods using a polymeric basic aluminum silicate sulfate. The treatments were at:    1) 8-9° C. at a pH of 6.65 to 6.93 and a concentration of 3-8 ppm Al2O3,    2) 25° C. and at a pH of 7.14-7.38 at a concentration of 3-8 ppm Al2O3; and
U.S. Pat. No. 6,165,369, patented Dec. 26, 2000, by General Chemical Corporation, which teaches a water treatment method that includes the use of polyaluminum silicate sulfate in conjunction with finely-divided, acid-insoluble solid particles.
Other patents of interest include the following:                U.S. Pat. No. 3,171,802, patented Mar. 2, 1965 by A. H. Rice;        U.S. Pat. No. 3,453,207, patented Jul. 1, 1969 by Allied Chemical Corp;        U.S. Pat. No. 3,506,570, patented Apr. 14, 1970 by R. F. Wukusch;        U.S. Pat. No. 3,617,569, patented Nov. 2, 1971 by The Dow Chemical Company;        U.S. Pat. No. 3,607,738, patented Sep. 21, 1971 by Nalco Chemical Company;        U.S. Pat. No. 3,655,552, patented Apr. 11, 1972 by Calgon Corporation;        U.S. Pat. No. 3,964,998 patented Jun. 22, 1976 by The South African Invention Development Corporation;        U.S. Pat. No. 4,029,575 patented Jun. 14, 1977 by Ewing Engineering Company;        U.S. Pat. No. 4,043,910, patented Aug. 23, 1977, by Allied Colloids Limited;        U.S. Pat. No. 4,049,545, patented Sep. 20, 1977 by P. J. Horvath:        U.S. Pat. No. 4,076,615, patented Feb. 28, 1978 by Batelle Pacific N.W. Laboratories;        U.S. Pat. No. 4,167,479, patented Sep. 11, 1979 by F. Besik;        U.S. Pat. No. 4,209,396 patented Jun. 21, 1980 by Filters International;        U.S. Pat. No. 5,271,848, patented Dec. 21, 1993 by R. W. Smith et al;        U.S. Pat. No. 5,364,529, patented Nov. 15, 1994 by Level Valley Dairy Company;        U.S. Pat. No. 5,759,401, patented Jun. 2, 1998 by Elf Atochem;        U.S. Pat. No. 5,853,588, patented Dec. 29, 1998 by Polytechnic University; and        U.S. Pat. No. 5,876,606, patented Mar. 2, 1999, by University of Waterloo.        
Other non-patent literature includes:
The publication by Gao et al. in Water Research 36(2002) 3573-3581, on pages 38 to 46 taught that PASiC and PAC could be used as a flocculent agents for water treatment. While there is a mention of PASS (poly-aluminum silicate sulfate) there is no teaching of its use in treating phosphorus-containing water.
The publication of Boisvert et al. in Water Research 31(1997) 1939-1946 focuses on the mechanism of flocculation and does not address the performance with respect to low phosphorus levels in the residual wastewater.
Additionally, Boisvert does not disclose or suggest a stage of filtering the water.
Finally, Boisvert concludes that alum performs better that PASS, (poly-aluminum silicate sulfate) which should lead a person skilled in the art away from trying PASS (poly-aluminum silicate sulfate).
In some existing, known, multi-stage physical/chemical and/or biological wastewater treatment systems, wastewater is processed by a combination of treatment technologies. For example, it is first introduced into a primary settling tank to allow the sedimentation of significant solids from the liquid. Thereafter, the wastewater is introduced into a series of one or more biological reactor tanks, wherein the wastewater is agitated in the presence of aerobic bacteria. One existing technology used for treating the wastewater at this stage, hereafter used as an example, is the employment of rotating biological contactors (RBC), i.e., large rotating disks serving as a support to a fixed-film of bacteria.
According to known procedures, a flocculent can be added to the wastewater while it is in the biological reactor. In some prior art systems, the addition of flocculent occurs when the wastewater has been partially, but not fully treated in the biological reactor, e.g., in the third stage of a four stage RBC. Because the biological reactions using aerobic bacteria must be carried-out at a pH level around 6.8 or higher, the coagulant/flocculent according to this known procedure is introduced into wastewater which has been adjusted, using another chemical, in order to have a pH of, for example, of 6.8 to 7.6.
Thus, in existing systems, a portion of the phosphorus present in wastewater is used for microbial metabolism and another portion precipitates-out and forms flocs in the biological reactor. Some of this precipitate in some systems is removed from the last stage of the biological reactor and returned to the primary settling tank where it is allowed to accumulate. This is done in order to minimize the amount of precipitate that is fed to the next stage in the wastewater treatment process.
In these existing systems, the wastewater leaving the biological reactor is typically fed to a second, quiescent, settling tank which is not subject to mechanical agitation. There, further settling of precipitated phosphorus is allowed to occur. The sludge from the bottom of such second (or final) settling tank is periodically pumped back to the primary settling tank where the phosphorus-enriched sludge accumulates. Pumping of the sludge from the secondary to the primary settling tank is optional.
The removal of phosphorous to very low levels with any degree of consistency has been problematic. In one aspect, the invention strives for consistent phosphorous removal of to such low levels, for example <0.03 mg/l.
It is thus recognized that there is a need for treatment processes, methods and systems for lowering the concentration of phosphorus present in wastewater that is effective, does not significantly put other pollutants into the water and uses materials that are readily and economically available. This invention addresses this need.
The present invention can be advantageously combined with various types of wastewater treatment systems including Rotating Biological Contactors.
The invention in its general form will first be described, and then its implementation in terms of specific embodiments will be detailed with reference to the drawings following hereafter. These embodiments are intended to demonstrate the principle of the invention, and the manner of its implementation. The invention in its broadest sense and more specific forms will then be further described, and defined, in each of the individual claims which conclude this Specification.